Production of tin plate



March 18, 1969 v, ups s ET AL 3,433,720

PRODUCTION OF TIN PLATE Filed Jan. 5, 1966 l; I I I l I 800 I000 I200 I400 I600 Line Speed, H/min.

fi l l l I I Reflow Temperature F INVENTORS. VISVALDIS KUPSIS A By ROYSTON P MORG N United States Patent 3 Claims ABSTRACT OF THE DISCLOSURE The method of making acid electrolytic tinplate which includes the step of chemically treating the metal to be treated with a solution of ferric chloride prior to its introduction to the plating unit and a postplating step of subjecting the treated metal to a tin reflow cycle to 1mprove the corrosion resistance (Alloy Tin Couple value).

This invention relates to the production of acid electrolytic tin plate and is especially directed to an improved treatment for preparing a ferrous base in the form of an elongated strip to receive a tin coating by electrolytic deposition from an acidic bath of tin thereon followed by a tin reflow step in which a portion of the tin coating in contact with the base becomes combined with the latter to form a layer or deposit of an iron-tin alloy between the outer coating of substantially pure tin and the ferrous metal base usually containing fractional percentages of elements other than iron but being composed primarily of that element.

It has been a practice in the production of acid electrolytic tin plate, especially those of the lighter grades carrying 1 lb. or less of tin per base box of plate, equivalent to 112 sheets each 14" x 20" in surface area exclusive of edges, to pass the ferrous base sheet or strip, after final rolling to gauge and alkaline cleaning and rinsing, through a pickle solution of sulfuric acid (H 50 and following subsequent final rinsing to remove excess acid into the plating unit usually comprising a plurality of acid electrolytic cells for deposition on the strip of a tin coating of predetermined thickness, the strip usually being passed through the cells at speeds of up to 1800 ft./ min. After receiving the tin coating, which may be of differential thickness on opposite sides of the strip, the coated strip is passed at a like speed through a tin reflow unit where the coating is subjected to brief heating as hereinafter more fully explained. This produces, especially in the lighter grades of tin plate now in demand, an alloy layer r between the underlying base metal and the outer tin layer or film which is discontinuous in that it is somewhat porous or in isolated but frequently occurring points so thin as to be of little or no significance and in consequence where pores or substantial voids in the alloy layer happen to register with voids or pores in the overlying tin layer the base metal is exposed and on contact of, for example, foodstuff content of a can made from the plated metal, an electrolytic couple may be established with consequent rapid corrosion of the metal.

It is therefore a principal object of this invention to provide a novel procedure for preparing a ferrous base, more specifically a steel strip of indeterminate length for introduction to an acid electrolytic tin plating unit for reception therein by known methods of an improved tin coating on its surfaces which, following postplating treatments including a tin reflow operation, imparts improved corrosion resistance to the coated metal as compared with like strips coated withan equal weight of tin per unit area by known acid tinning'procedures.

A further object is to provide a method of acid electrotinplating a ferrous metal strip to improve its resistance to corrosion (as compared with strips plated with corresponding amounts of tin per unit area by other methods) which includes utilization of a solution of ferric chloride (FeCl for chemically treating, as opposed to electrochemically treating (in the absence of an electrolytic current), the previously cleaned surface of the strip just prior to its introduction to the plating unit and other incidental operations some of which are well known in the art and all of which contribute to the attainment of the said result.

Other objects, purposes and advantages of the invention will hereinafter more fully appear or will be understood from the following description of its practice in which reference will be had to the accompanying drawing wherein there are depicted in:

FIG. 1 is a graphic representation of the improvement in ATC values of ferrous metal strip acid tinplated in accordance with the invention as compared with those of like strip plated in accordance with the best known prior acid method for producing a tinplated strip having a plating of substantially like thickness thereon;

FIG. 2 is a generally similar representation of the results obtained from testing for ATC values generally corresponding specimens subjected to a plurality of different reflow temperatures.

The ATC (Alloy Tin Couple) values to which reference has been made are determinations with which those skilled in the art are already familiar but to avoid any possible misunderstanding a brief reference to the manner in which they are reached will now be made. The ATC values are a reflection of the capacity of tin plate identical in all respects with the specimens tested under controlled conditions to resist corrosion when made into a container filled with certain foodstuifs (e.g., grapefruit juice) and stored. ATC values are arrived at in the following manner: Tinplated specimens, plated in accordance with the procedure chosen are subjected to an electrolytic stripping treatment to remove free (unalloyed) tin but to leave exposed and unimpaired the layer of iron-tin alloy formed between the base metal and the tin during the reflow cycle. The stripped samples are placed in a cell containing suitably modified grapefruit juice (see Corrosion, vol. 17, page 113, February 1961) and a pure tin electrode. The cells are then stored at a constant temperature of F. for 20 hours, after which time the current flowing between the tin electrode and the specimen is measured. This current, expressed in microamperes per square centimeter, is known as the ATC (Alloy Tin Couple) value. A low current (low ATC value) is indicative of a higher corrosion resistance.

The tin reflow cycle normally comprises a brief heating 3 step during which the tin coating is brought to a temperature not in excess of 650 F. and maintained at that temperature for a period of time of the order of one second or less prior to cooling or quenching; as indicated above effects of different reflow temperatures with all other factors equal are depicted in FIG. 2.

In accordance with the invention ferrous strip destined to become electrolytic tin plate after a final rolling which reduces it to a finished thickness is normally packaged in coils of hundreds of lineal feet each and the electrolytic tin plating line designed to accommodate such coils progressively includes uncoilers on which a number of the coils may be mounted, a shear, welding equipment, looper, cleaning and pickling tanks, scrubber, plating tanks, reflow unit, quench tank and finishing and receiving apparatus in which the plated strip may be oiled, inspected, sheared and delivered either as discerptible plates or sheets of predetermined uniform length and breadth or recoiled in discrete coils of predetermined length for delivery to the consumer. All this apparatus is Well known in the art and while the practice of our invention is not restricted to its utilization it will be normally found convenient in attaining the objectives of the invention to employ like or generally corresponding units operating in their several normal respective schedules with the exceptions hereinafter noted.

Thus in a typical employment of the invention in one of the cleaning and pickling tanks, if more than one are included in the line, or in the pickling tank if but one is employed, but preferably at a position to affect the strip just prior to its passage through the scrubber and into the plating tanks, we dispose in lieu of or in addition to the usual sulfuric acid solution (H 80 a volume of an aqueous solution of ferric chloride (FeCl preferably at a concentration of about 5%-10% FeCl although these limits are not to be deemed critical and may be departed from if desired especially as modification in other respects are utilized. Hence while at an operating speed of 1500 lineal feet of strip per minute and the ferric chloride bath maintained in a pickling tank at a temperature of 70160 F. the ferric chloride concentration should be within or not greatly depart from the range stated, but when other methods of effecting contact between the ferric chloride solution and the strip, especially at different temperatures and other speeds, reflecting greater or less periods of strip exposure to the chemical action of the solution are employed, the concentration of ferric chloride in the solution should be modified accordingly.

As a base metal open hearth steel of substantially the following composition has been found eminently suitable for utilization in the production of sheets or strip to be tinplated in accordance with the invention:

Elements Percent Carbon maximum .12 Manganese .35.55 Phosphorus maximum 0.15 Sulphur do .015 Sulphur do .040 Silicon do .010 Copper do .06 Tin do .020 Nickel do .04 Chromium do .06 Molybdenum do .05

Utilizing this steel as the base metal and tinplating it in accordance with the invention to a coating thickness corresponding to about 0.50 lb. of tin per base box (#50 plate) yielded specimens which when subjected to ATC determinations afforded results indicated in FIG. 1 wherein line A is the approximate average or mean ATC value of a number of specimens taken from different strips so plated using a 6% ferric chloride bath and reflow temperature of 540 F. but at several different linear speeds in the plating line, i.e. from about 800 to about 1600 ft./ min., the individual results showing variations in ATC values, in microamperes per square centimeter from about 0.05 or less to not in excess of about 0.10; equivalent specimens from strips plated in accordance with prior practices at the same reflow temperature and like speeds but with normal H pickling prior to plating as shown by line B were found to have ATC values no lower than about 0.10 ranging upward to a high of about 0.20 representing about one-half as effective corrosion resistance (as measured by the ATC test) as compared with plating produced in accordance with the invention.

Other properties found in the latter plating include uniformly low pickle lag (10 seconds or less), iron solution values not in excess of 20 micrograms of iron, highly acceptable crystal size (ASTM #9 or coarser) and alloy color (light gray) while as demonstrated by line A in FIG. 2 the ATC values are consistently below those for sulfuric acid pickled plate plotted in line B in FIG. 2 when specimens from plate subjected to reflow treatment at different temperatures in plating lines operating at 1200 ft./min. are tested. Thus at approximately 460 F. reflow temperature the ATC value of 0.15 compares with 0.35 for ordinary sulfuric acid pickled tin plate refiowed at the same temperature, and while the difierences are not so marked when refiow temperatures are increased, even at about 600 F. reflow temperature which is close to the maximum practical in view of the susceptibility of tin to oxidation at above 650 F., the ATC value of conventional tin plate is still twice that of plate produced in accordance with the invention (0.10:0.05).

Apart from the improved results attained by its practice, our invention is of advantage in that its use requires no substantive modification of tinning lines of ordinary construction and does not reduce the normal time of production of a tinned strip of given width and length nor save in the respects herein set forth, modification of the customary operations employed in the production of such strip.

Having thus described our invention, we claim and desire to protect by Letters Patent of the United States:

1. A method of producing acid electrolytic tin plate, which method comprises:

(a) subsequent to a cleaning treatment, subjecting a sheet of ferrous metal to the chemical action, of an aqueous solution of ferric chloride, in the absence of an electrolyte current in said solution;

(b) depositing, by means of an electroplating bath, a substantially continuous coating of metallic tin on the ferric-chloride chemically treated sheet;

(0) withdrawing said sheet from said bath;

(d) rapidly heating a coated surface of said sheet to a temperature not in excess of 650 F., maintaining said surface at approximately said temperature for about one-half to one second; and

(e) cooling said heated surface.

2. The method as defined in claim 1 in which the aqueous solution of ferric chloride contains about 5%10% of ferric chloride and the remainder is substantially pure water.

3. The method as defined in claim 1 in which said surface is heated to a temperature not less than 450 F.

References Cited UNITED STATES PATENTS 2,266,430 12/ 1941 Matthews et a1 15618 2,631,950 3/1953 Rosenfeld et al. 156-18 2,907,701 10/1959 Peart et al. 20434 XR 3,062,726 11/ 1962 Hill 204-40 XR 3,197,341 7/1965 Wallace 134-28 OTHER REFERENCES Miller et al.: Metal Progress, November 1949, pp. 692-695.

JOHN H. MACK, Primary Examiner.

W. B. VAN SISE, Assistant Examiner.

US. Cl. X.R. 

